1. Field of the Invention
This invention relates to a device for making pneumatic tires, and more particularly to a tire-making apparatus, in which tire tread rubber is separately formed in a toroidal or cylindrical form and the tire tread rubber is integrally bonded to the outer peripheral surface of a green case in the course of vulcanizing the latter. The green case, which is preformed on a separate former, comprises a pair of wire beads and a carcass extending across the two wire beads. The green case normally includes a peripheral rubber layer overlaid on the outer surface of the carcass along the circumference thereof. The toroidal tire-tread rubber may be bonded to the green case in a mold for shaping and vulcanizing the green case into a pneumatic tire.
2. Description of the Prior Art
In the conventional process of making pneumatic tires, a green case is formed by spreading and winding one or more rubberized cord carcass layers on a cylindrical metallic former, disposing annular beads at suitable positions on the rubberized cord layer, and applying a separately formed tread rubber layer on the outermost surface of the rubberized cord layer. The tread rubber layer is formed by extrusion. The green case is then removed from the metallic former and shaped in a metal mold into a toroidal form. After being shaped, it is vulcanized in the metal mold at a suitable temperature and pressure so as to provide the desired pneumatic tire.
To improve the performance or operating characteristics of pneumatic tires, various kinds of tread patterns have been proposed. In some of the tread patterns, the thickness of the tire tread rubber layer widely varies. Since the configuration of the tread rubber layer is greatly changed in the course of the aforesaid vulcanization in the metal mold, it has been difficult to provide the desired tread pattern on the extruded tire-tread rubber layer to be applied on the outer surface of the green case. To form a complicated tread pattern having a wide variation in its cross-sectional shape, the rubber material in the separately extruded tread rubber layer on the green case must be flowed over a considerably large distance, because the cross-sectional shape of an extruded tread rubber layer is substantially uniform throughout the tread rubber layer and the widely-varying cross-sectional shape cannot be made by extrusion. Such flow of the rubber material in the tread portion tends to cause either or both of the breaker and carcass layers to be distorted from the smooth regular toroidal carcass shape, so that undulations are caused in the carcass layer beneath the tread rubber layer.
In order to mitigate such difficulty, Canadian Patent No. 853,659, which was issued to The Dunlop Company Limited on Oct. 13, 1970, teaches a method of making a pneumatic tire by using a discontinuous ring made of segments which are separately prepared and joined to the outer periphery of a carcass through an assembling operation. This Canadian Patent, however, is rather complicated because a number of different tread segments must be formed, and they must be connected to one tire carcass in proper alignment with each other. U.S. Pat. No. 3,386,485, which was granted to Stanley R. Harrison et al. on June 4, 1968, and U.S. Pat. No. 3,425,475, which was patented to Kenneth A. Hoy on Feb. 4, 1969, teach the art of making tubeless tires in which the need of assembling a tire tread on a carcass is completely eliminated. The tubeless tires of the aforesaid two U.S. patents have shortcomings in that they can be used only at a comparatively low pressure, and it is rather difficult to form deep tread patterns on the tread portion of such tubeless tires.
Therefore, an object of the present invention is to provide an improved method and apparatus for making pneumatic tires with a complicated tread pattern.